Wireless speed problems

I'm trying to determine why my speed keeps dropping. I'm paying for 16mb down with Charter Communications. I'm eagerly waiting to upgrade to the 60 once its in my area. I use a Linksys WRT54GX2 router and I have one ethernet connection to it, a 360, Wii, my blackberry and two wireless laptops. I speed test on "testmy.net" and my speed is usually close to the 16. When I connect directly to my modem the test is pretty steady. The problem I'm noticing is that my speed will drop down to 250kb out of nowhere and slowly go back up. As far as security, I do not have my ssid broadcasted, i don't use WEP or any pass codes, but I my connect by mac filter only in my router.

So I'm wondering...

Is there anything I can do to stop my speed from going up and down the way it does?